(VC) Improve segmentation start behavior

[csparker247]

One last low priority thing would be to make the enter key work as 'start' in the Ending Slice box, the way enter works in the choose slice box.

Originally posted by @hariseldon137 in #28 (comment)

Related to the above request, the behavior of tabbing through the segmentation options can be improved. For example, hitting tab while focused on the Ending Slice entry box does not transfer focus to the Start button.